Specific properties of material intermediate or end products such as the color purity, the impurity content, or the degree of whiteness can be detected using visible light and optical sensors and used for purposes of sorting out.
However, frequently, simple detection of the surface color cannot be used, since there is not a sufficient correlation between the surface color and the desired property, such as the content or proportion of a specific mineral. Alternatively, one can then use costly detection methods like X-ray fluorescence analysis (in this regard see U.S. Pat. No. 7,763,820 B1, for example) or NIR (near infrared) spectroscopy.
Basically speaking, it is known that mineral deposits, whose decomposition product consists of minerals or mineral phases, exhibit fluorescence properties. For instance, chalcedony exhibits green fluorescence, fluorite exhibits blue or yellow fluorescence, and calcite exhibits red fluorescence. With the exception of a few minerals such as the tungsten ore scheelite, fluorescence is not an inherent property of the individual minerals, but rather is dependent on the origin and thus is for the most part deposit-specific. A specific fluorescence property is essentially defined by the crystal structure and crystal lattice defects in which activators such as rare earth elements or transition metals are incorporated.
For this reason, fluorescence is also used for analysis of minerals, mineral resources, or mineral-containing rocks, for example, in laser-induced fluorescence (LIF). This method is also a spectroscopic method and for this reason would be too costly and time intensive for use in industrial sorting.
It is also known that plastics have fluorescence properties. US 2013/274914 A1, for instance, concerns the sorting of plastic parts, including fluorescent plastic parts, by means of radiation. DE 10 2010 030908 A1 in turn shows a method for classification of objects contained in seeds, where the fluorescence property is utilized.